To enhance the understanding of the discussion that follows, the abbreviations and terms listed below will have the definitions as shown, the meaning and significance of which will be readily apparent to one skilled in the art of circuit board capacitor structures:
ADC—Analog to digital converter;
BEOL—Back end of line;
CA—Tungsten contact between metal and polysilicon;
Csub—Adjustable capacitor;
DAC—Digital to analog converter;
FEOL—Front end of line;
MIMCAP—Metal-insulator-metal capacitor;
MOS—Metal oxide silicon;
RF—Radio frequency;
VNCAP—Vertical native capacitor.
On-chip capacitors are critical components of integrated circuits that are fabricated on silicon semiconductors. These capacitors are used for a variety of purposes including bypass and capacitive matching as well as coupling and decoupling. For example, FIG. 1 illustrates three different silicon semiconductor chip functional capacitor structures: (a) a by-pass capacitor structure BPC; (b) an AC-coupling capacitor structure ACCC; and (c) a reactive capacitor structure RC for high frequency matching. More particularly, in the by-pass capacitor structure BPC in FIG. 1(a), a capacitor 100 is configured to bypass AC noise signals 103 from a power supply 101. As is well known, a power supply signal 102 from a power supply 101 may include AC noise signals 103, including noise signals 103 from other neighboring circuits (not shown). It is preferable to remove the AC noise signals 103 from the power supply signal 101 prior to supply of power to the circuit structure 105. Accordingly, the bypass capacitor 100 is provided to flow the AC noise signals 103 into ground G and provide a clean DC power signal 104 to the circuit 105.
FIG. 1(b) illustrates an AC-coupling capacitor structure ACCC to de-couple a DC signal 107 and couple an AC signal 109 into a circuit input port 110. By locating a DC de-coupling/AC coupling capacitor 106 in series between two ports 108 and 110, the capacitor 106 blocks DC signal 107 flow, thereby allowing only an AC signal 109 to pass into the circuit 110. And FIG. 1(c) illustrates a reactive capacitor structure RC, wherein a capacitor 111 provides a high frequency capacitive component for a circuit input 113, the signal coupling at a high frequency region based on characteristic impedance matching to reduce reflected power between ports 114 and 115.
The design and implementation of by-pass capacitor, AC-coupling capacitor and reactive capacitor structures on silicon semiconductor chips may be dependent upon one or more symmetrical structural, target circuit quality and low parasitic resistance performance characteristics. In particular, a bypass capacitor structure is typically required to provide a highest capacitance possible relative to the physical structure of the circuit and device. However, the reactance resistance of the bypass capacitor is generally required to be as low as possible for a target AC noise signal frequency. More particularly, reactance resistance R_cap(f) may be computed through the following Equation 1:R_cap(f)=1/(2*pi *f*C);   Equation 1wherein pi is a constant, the ratio of a circle's circumference to its diameter (i.e. about 3.14); f is the frequency of the AC flowing through the circuit; and C is a capacitance value of the capacitor element in the circuit, for example capacitor 100 in FIG. 1(a).
It is known to use a metal oxide silicon (MOS) capacitor, or MOSCAP, for the capacitor element 100. However, MOSCAP capacitors require large chip area footprints in integrated circuits (IC). Accordingly, prior art design requirements typically result in requiring large semiconductor chip footprint areas or real estate for a bypass capacitor structure, resulting in high production costs and reduced semiconductor chip area availability for other circuit structures. As the production cost of an IC is generally proportional to the real estate required, it is desired to reduce IC chip costs by reducing the footprint required for a MOSCAP structure.
Moreover, current leakage during a semiconductor circuit's idle mode is known to result in increased power consumption. Silicon semiconductor chip capacitor structures usually require large MOSCAP capacitor structures in order to avoid current leakage problems.
What is needed is a method and structure for providing high density, high yield on-chip capacitor structures for integrated circuits and, more particularly, for silicon-based semiconductor chips.